In the field of nuclear medicine, a patient is injected with a radiopharmaceutical substance, and images of internal structures or functions of the patient's body are generated by an imaging system that detects radiation emitted by the substance. The imaging system typically uses one or more scintillator-based detectors to detect the radiation. A computer system controls the detectors to acquire data based on the detected radiation and processes the data, using a technique known as tomography to generate the images. Nuclear medicine imaging techniques include Single-Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET), or "coincidence" imaging.
One factor that can affect image quality in nuclear medicine imaging systems is the non-uniform attenuation of radiation within the body. Non-uniform attenuation tends to distort and introduce artifacts in images generated by the imaging system. The effects of attenuation can be especially significant in cardiac studies due to attenuation caused by the thorax. Most modern nuclear medicine systems provide correction for non-uniform attenuation. One common technique is to perform a transmission scan of the patient either before or concurrently with the emission scan. During the transmission scan, radiation is transmitted through the patient's body from an external radiation source to one or more detectors of the imaging system. The data acquired based on the transmission scan represents an "attenuation map" of the patient, which can be used to correct the emission data for the effects of attenuation.
An undesirable consequence of the above-mentioned attenuation correction technique is that the patient is exposed to additional radiation. Consequently, it is desirable to limit the amount of this additional radiation. One way of doing this is to limit the duration of the transmission scan. However, a certain minimum amount of transmission radiation must be used to enable the detectors to acquire enough data to form a usable attenuation map. One possible approach is to use the same amount of transmission radiation regardless of the size and shape of the patient. However, with such an approach, patients with very small bodies would tend to be exposed to more transmission radiation then is necessary. Similarly, a transmission scan of a very large patient would tend to yield insufficient transmission data.
With certain prior art nuclear medicine imaging systems, a medical technician decides what the duration of the transmission scan should be, based on the size and shape of the patient. This decision may be made on the basis of recommendations from the manufacturer of the imaging system. However, because the decision is based on human judgment and approximations, it is inherently subject to error. U.S. Pat. No. 5,629,971 of Jones et al., which is assigned to the assignee of the present invention, describes a technique for computing a desired duration of the transmission scan for each patient, based on results of a brief transmission "pre-scan". However, the described technique, which requires complete motion of the radiation line source across the field of view of the detector, tends to expose the patient to more radiation during the transmission pre-scan than is desirable. In addition, the described technique fails to take into consideration the effects of emission contamination in the transmission pre-scan. Emission contamination may result when the transmission scan is performed after injection of the radiopharmaceutical, and the emission and transmission energy ranges overlap, as is often the case.